Words with Friends/Lexulous are my constant go-to games.
It hurts me to admit this, but Farmville as well. Even though it's not an iPad app and it crashes enough to make me want to throw it into next week.
Angry Birds is something I always come back to, as is Tesla Wars (which REALLY surprised me with how fun it was - one of the best free apps out there)
Amateur Surgeon recently hooked me as did Solipskier.
Unfortunately I haven't touched Flight Control in WELL over a month, which is a shame, twas probably my first purchase on the iPad
There's also Spider which I find my way to